A thick film dielectric structure provides for superior resistance dielectric breakdown as well as a reduced operating voltage in comparison to thin film electroluminescent (TFEL) displays, as exemplified by U.S. Pat. No. 5,432,015. The thick film dielectric structure when it is deposited on a ceramic substrate tends to withstand higher processing temperatures than TFEL devices, which are typically fabricated on glass substrates. This increased high temperature tolerance facilitates annealing of phosphor films at higher temperatures to improve their luminosity. However, even with this enhancement, thick film electroluminescent displays have not achieved the phosphor luminance and color coordinates needed to be fully competitive with cathode ray tube (CRT) displays, particularly with recent trends in CRT specifications to higher luminance and higher color temperature. Some improvement may also be realized by increasing the operating voltage of the displays, but this increases the power consumption of the displays and decreases the reliability and increases the cost of driving electronics for the displays.
A high luminosity full color electroluminescent display requires that the thin film phosphor materials used for the red, green and blue sub-pixels be patterned so that the emission spectrum for each color of pixel is tailored to minimize the attenuation resulting from the use of optical filters that are needed to achieve the required color coordinates for each sub-pixel. For relatively low-resolution displays, the required patterning can be achieved by depositing the phosphor materials through a shadow mask. For high-resolution displays, however, the shadow mask technique does not provide adequate accuracy, and photolithographic methods must be employed. Photolithographic techniques, as exemplified in published PCT patent application WO 00/70917 of Wu et al., require the deposition of photoresist films and the etching or lift-off of portions of the phosphor film to provide the required pattern. Deposition and removal of photoresist films and etching and/or lift-off of phosphor films typically requires the use of solvent solutions that contain water or other protic solvents. These solutions may degrade the properties of phosphor materials, such as strontium sulphide that are subject to hydrolysis.
Traditionally, cerium-activated strontium sulphides for blue and manganese-activated zinc sulphides for red and green have been the phosphor materials of choice for full color electroluminescent displays. The optical emission from these phosphor materials must be passed through an appropriate chromatic filter to achieve the necessary color coordinates for red, green and blue sub-pixels, resulting in a loss of luminance and energy efficiency. The manganese-activated zinc sulphide phosphor has a relatively high electrical to optical energy conversion efficiency of up to about 10 lumens per watt of input power. The cerium-activated strontium sulphide phosphor has an energy conversion efficiency of 1 lumen per watt, relatively high for blue emission. However, the spectral emission for these phosphors is quite wide, with that for the zinc sulphide-based phosphor material spanning the color spectrum from green to red and that for the strontium sulphide based material spanning the range from blue to green. This necessitates the use of optical filters. The spectral emission of the cerium-activated strontium sulphide phosphor can be shifted to some degree towards the blue by controlling the deposition conditions and activator concentration, but not to the extent required to eliminate the need for an optical filter.
Alternate blue phosphor materials having narrower emission spectra tuned to provide the color coordinates required for blue sub-pixel have been evaluated. These include cerium-activated alkaline earth thiogallate compounds. Such blue phosphor materials tend to give good blue color coordinates, but have relatively poor luminosity and stability. Since the host materials are ternary compounds, it is relatively difficult to control the stoichiometry of the phosphor films.
Europium-activated barium thioaluminate provides excellent blue color coordinates and higher luminance, but it is also a ternary compound and stoichiometry is difficult to control. Vacuum deposition of phosphor film comprising this material from a single source target using sputtering or electron beam evaporation has not yielded films with high luminosity. Improved luminance of barium thioaluminate phosphors has been achieved by using a hopping electron beam deposition technique to deposit films from two source pellets. The stoichiometry of the deposited film is controlled using the relative dwell time of the electron beam impinging on each of the two source materials. However, this technique is not readily scalable to facilitate commercial production of large area displays and the process cannot be controlled to compensate for changes in the evaporation rates from the two sources as the deposition proceeds and the source pellets are depleted.
Another approach that has been adopted to improve the stoichiometry of the thioaluminate phosphors is to use more than one source for the deposition, but this approach requires added controls over the relative deposition rates for the different sources. The required relative evaporation rates must be calibrated for each specific piece of deposition equipment and the requirement for multiple sources constrains the design of the deposition equipment, generally adding to the cost of the equipment.